docs: def os_release(package, base='essex', reset_cache=False):
      '''
      Returns OpenStack release codename from a cached global.
  
      If reset_cache then unset the cached os_release version and return the
      freshly determined version.
  
      If the codename can not be determined from either an installed package or
      the installation source, the earliest release supported by the charm should
      be returned.
      '''
      global _os_rel
      if reset_cache:
          reset_os_release()
      if _os_rel:
          return _os_rel
      _os_rel = (
          get_os_codename_package(package, fatal=False) or
          get_os_codename_install_source(config('openstack-origin')) or
          base)
      return _os_rel